Absorbent papers are generally manufactured by processes which include suspending cellulosic fibers in an aqueous medium, then removing most of the water from the web by gravity or vacuum-assisted drainage, with or without pressing, followed generally by evaporation either on a drying fabric and/or a Yankee dryer. Manufacture also includes creping in many cases, wherein the cellulosic web is adhered to the surface of a cylindrical dryer, e.g., a Yankee dryer and thereafter separated from the Yankee dryer, typically with the aid of a creping blade. The resultant sheet is wound onto a reel. While paper derives structural integrity from the arrangement of the cellulosic fibers in the web, and also from hydrogen bonding that links the cellulosic fibers to one another, many desirable aesthetic and physical properties of absorbent paper products are influenced by creping from a dryer; for example, creping from a Yankee generally enhances at least one of bulk (and corresponding absorbency), stretch, and softness of the resultant paper product, in part, through disruption of hydrogen bonds between fibers. A creping adhesive is used to increase the effectiveness of the creping operation by adhering the web to the Yankee as well as aiding in the transfer of the web to the drying surface. Creping adhesives also increase drying efficiency by promoting contact between the dryer surface and the paper web and thus are used even in cases where the product is peeled (i.e., little reel crepe) rather than creped from the dryer surface.
Historically, common classes of thermosetting adhesive resins that have been used as Yankee dryer adhesives include poly(aminoamide)-epihalohydrin polymer (PAE) resins, such as those polymers sold under the tradenames KYMENE® and CREPETROL® (Ashland, Inc.), ULTRACREPE® (Process Application Ltd. “PAL”), BUBOND® (Buckman Laboratories Inc.). Modern manufacturing processes which use Yankee drying such as through-air drying processes, low-compaction pneumatic dewatering processes and newer fabric-creping or vacuum dewatering processes which do not involve wet-pressing a relatively wet web on a felt to a Yankee dryer typically require an adhesive coating which is both relatively durable as well as rewettable. The requirement of promoting transfer to a Yankee of partially dried, moist webs with a patterned fabric in the transfer nip is particularly challenging when a spray softener is applied to the web prior to transfer to the Yankee as is discussed further herein.
Rewettable PAE/polyvinyl alcohol adhesives are disclosed in U.S. Pat. No. 4,501,640 to Soerens et al. This class of adhesives offers superior adhesion as well as rewettability. It has been postulated that this particular admixture as a creping adhesive is particularly effective for at least two reasons. The first reason is that polyvinyl alcohol is a rewettable adhesive. Rewettability is an important characteristic of creping adhesives since only very small amounts of adhesive are added per revolution of the creping cylinder; provided the newly added adhesive wets the existing adhesive layer, all of the adhesive on the cylinder becomes available to adhere to the web. While the polyamide adhesive is relatively durable, if used by itself it will eventually irreversibly harden and therefore lose its effect as an adhesive. However, by diluting this component with polyvinyalcohol, wettability is greatly improved and the effective life of the adhesive layer on the creping cylinder is extended. The second reason proposed for the success of PAE/polyvinyl alcohol creping adhesives is the cationic nature of the polyamide resin makes it a very specific adhesive for cellulose fibers.
U.S. Pat. No. 7,608,164 to Chou et al. refers to polyvinyl alcohol copolymers which may be used in creping compositions with PAE resins; however, no examples are provided. See Column 8, lines 24-49. See also, U.S. Pat. No. 7,404,875 to Clungeon et al. Col. 1, line 66 to Col. 2 line 35. It will be appreciated by one of skill in the art that there are a large number of known copolymers of polyvinyl alcohol. See United States Patent Application Publication 2002/0037946 of Isozaki et al. which discloses a listing of polyvinyl alcohol copolymers, paragraph [0015], page 2 which mentions comonomers such as acrylic acid, salts thereof and acrylate esters such as methyl acrylate, ethyl acrylate, n-propyl acrylate, isopropyl acrylate, n-butyl acrylate, isobutyl acrylate, tert-butyl acrylate, 2-ethylhexyl acrylate, dodecyl acrylate and octadecyl acrylate; methacrylic acid, salts thereof and methacrylate esters such as methyl methacrylate, ethyl methacrylate, n-propyl methacrylate, isopropyl methacrylate, n-butyl methacrylate, isobutyl methacrylate, tert-butyl methacrylate, 2-ethylhexyl methacrylate, dodecyl methacrylate and octadecyl methacrylate; acrylamide and derivatives thereof such as N-methylacrylamide, N-ethylacrylamide, N,N-dimethylacrylamide, diacetone acrylamide, acrylamidopropanesulfonic acid or salts thereof and acrylamidopropyldimethylamine or salts or quaternary ammonium salts thereof; methacrylamide and derivatives thereof such as N-methylmethacrylamide, N-ethylmethacrylamide, methacrylamidopropanesulfonic acid or salts thereof, methacrylamidopropyldimethylamine or salts or quaternary ammonium salts thereof and N-methylolmethacrylamide or derivatives thereof; vinyl ethers such as methyl vinyl ether, ethyl vinyl ether, n-propyl vinyl ether, isopropyl vinyl ether, n-butyl vinyl ether, isobutyl vinyl ether, tert-butyl vinyl ether, dodecyl vinyl ether and stearyl vinyl ether; N-vinylamides such as N-vinylpyrrolidone, N-vinylformamide and N-vinylacetamide; allyl ethers having a polyalkylene oxide side chain; nitrites such as acrylonitrile and methacrylonitrile; vinyl halides such as vinyl chloride, vinylidene chloride, vinyl fluoride and vinylidene fluoride; allyl compounds such as allyl acetate and allyl chloride; maleic acid or salts or esters thereof; vinylsilyl compounds such as vinyltrimethoxysilane; propenyl acetate and the like.
Creping adhesives, while much improved over the years, need further development as requirements for more adhesive strength and more rewettability are made in connection with new processes and increased machine speeds. Such properties are exceedingly difficult to achieve especially because the adhesive must remain soft and release the web at the dry end of the Yankee.
Wet tack is a measure of the ability of the adhesive coating on the drying cylinder to adhere a wet cellulosic web to the cylinder. The level of adhesion of the cellulosic web to the drying cylinder is generally important as it relates to transfer of the web from a creping fabric to the drying cylinder, as well as control of the web between the dryer and the reel upon which the web is wound. If the web is not sufficiently adhered to the drying cylinder, it may blister or become disengaged from the drying cylinder. Poorly adhered webs are difficult to control and can cause wrinkles during the winding of the web to the parent roll. Further, poorly adhered webs can reduce the potential stretch, bulk and softness properties of the web provided by creping.
Using spray softeners in a tissue making process is highly desirable since the softener can be applied directly to the surface of the sheet where softness is desired instead of being added to the furnish in the wet-end of the papermachine where the softener is dispersed throughout the entire web. The softener is thus more effectively used to achieve the desired effect and less likely to raise manufacturing issues associated with insufficient tensile, since most softeners act as debonders as well. Spray softeners, however, are typically surface active agents and further exacerbate adhesion problems. It has been found that the creping adhesives of the present invention are surprisingly tolerant of spray softeners in papermaking processes.
The level of adhesion of the cellulosic web to the dryer is also important as it relates to drying efficiency. Higher levels of adhesion generally reduce the impedance to heat transfer causing the web to dry faster, thereby enabling more energy efficient, higher speed operation.
Conventional creping adhesives, including PAE/polyvinyl alcohol compositions tend to develop a hard coating which is less rewettable after undergoing the extensive drying required for low moisture creping and removal from the dryer. This hard coating results in a loss of adhesion and also results in blade vibration (chatter), which can cause non-uniform creping, blade wear, and, in extreme cases, damage to the Yankee dryer cylinder surface. Thus, there is a great demand for a creping adhesive that remains soft and rewettable under drying conditions encountered in low moisture creping.
As the demand for softer tissue products continues, the limitations of the current creping adhesive coating packages have become apparent, especially in connection with processes including transfer to a Yankee from a patterned fabric and processes where sprayed-on softeners are employed. The alternative adhesive products of the invention are more effective than conventional adhesives in achieving excellent transfer at the pressure roll and high Yankee adhesion while maintaining a soft coating at low moistures and tolerance to spray softeners.